1. Field of the Invention
This invention relates generally to document reading in which it is desired to determine the presence or absence of a mark which is indicative of certain coded information, and pertains more especially to apparatus for discriminating between intended marks representing selected answers or other coded information contained on the document as contrasted with unintended ones that have been incompletely or inadequately erased.
2. Description of the Prior Art
Examination answer sheets of the envisaged types are now rather widely used and need not be described in any great detail. However, it should be borne in mind that discrete areas are provided on a translucent sheet of paper. The answer sheet, quite obviously, is designed for a given type of examination and the questions are multiple choice questions, sometimes being true or false and at other times requiring the selection of one or more answers where a number of possible answers can be chosen. Frequently, however, the examinee changes his mind after marking a particular area with a lead pencil, attempts to erase the mark, but does so in a manner such that the mark is still discernible by conventional grading apparatus. Consequently, correct answers can go undetected by the usual type of photoelectric scoring apparatus, or an answer may be ruled out by the machine because it senses two responses which calls for a disqualification of that particular answer.
One prior art method that has been commonly employed involves the use of a two-threshold techniques for determining the presence of a mark. The lowest threshold voltage is used to indicate whether there is a mark (light or dark or even possibly an erasure). The higher threshhold voltage is then used to indicate whether the mark is a dark mark and not an erasure or light mark. Thus, when utilizing the two-threshold method alluded to above, there can be only three parameters obtained for a mark: (1) a parameter indicating that there is no mark, (2) that there is a light mark, or (3) that there is a dark mark.
For instance, if on an answer sheet having a grid of five responses, there is an erasure on one so-called "bubble" or response area and a light mark in another bubble or response area, the result would be that the apparatus would indicate that there is no mark in either one of these bubbles or areas, that a light mark exists in both of them, or the grading machine would come up with the result that there is no mark in one instance and a light mark in the other. Accordingly, with this particular prior art method, whenever there would be an erasure and a light mark within the grid of five the machine could signify that there is a light mark in both bubbles or response areas and the sheet would be diverted into a rejected pile for manual editing. Such a system has resulted in a fairly high percentage of sheets that have to be edited by hand, this being approximately 5%. Not only does this particular prior art method entail greater manual editing, but if a change in the threshold level at which the equipment is to start picking up or responding to lighter or less opaque marks, the change had to be accomplished by manually adjusting or "tweeking" the potentiometer for each photoelectric cell, this being quite time-consuming.
The other commonly used method with which we are familiar has been called the darkest mark approach, this being where the circuitry acts to take off or pick-up the highest voltage value seen as far as a particular bubble or response area is concerned in the grid being sensed. As pointed out above, a grid usually comprises five response areas, but this number can be varied, a grid of only two being involved in the case of true and false questions. In such a system, there is a salient disadvantage stemming from the fact that there is little edit capability. Should an erasure appear as a mark slightly darker or more dense than a light mark, the equipment under these circumstances will pick the erasure instead of a true answer. Stated somewhat differently, the examinee in such a situation erases a mark and then, to avoid penalty for an incorrect answer, he does not darken another area in substitution for the bubble he has erased. Unfortunately, with this type of prior art machine, he ends up being penalized anyway.